The present invention relates generally to improvements in optical bar code scanners. More particularly, the invention relates to methods and apparatus for dual video channel data recovery and advantageous thresholding techniques which may be employed in conjunction therewith.
Optical scanners, and especially bar code scanners, are in wide use in a variety of applications, and serve to increase the accuracy and efficiency of data acquisition wherever they are used. In typical use, a standardized bar code label is affixed to an object, the bar code carrying an identifying number for the object. The object is passed along a scan field, where the bar code reflects light emitted by a laser or other light source, and reflects the light back to a data acquisition channel. The data acquisition channel decodes the reflected pattern and uses the decoded information to retrieve data associated with the object.
Objects with bar code labels are typically moved across the scan field manually, at varying rates of speed. Further, the distance from the scanner window will typically vary from scan to scan. The further the label is from the window the higher the frequency in the captured signal for the bar code label. Therefore, the frequency of the light modulation captured by a scanner may vary from one scan to another, and even within the same scan. Moreover, it is desirable to be able to use bar code labels of varying sizes with the same scanner. The use of different-sized bar code labels also varies the frequency of the modulated light pattern captured by the capturing mechanism. It is therefore desirable to maximize the frequency range over which the data recovery of the bar code scanner operates.
Bar code labels are also subject to certain paper and printing imperfections, which cause interference with a successful scan. Labels with numerous imperfections are typically read at one threshold extreme or the other. For example, a high level of noise resulting from paper and printing imperfections may be thresholded out using a higher DC threshold level. However, the risk that is run is that this high threshold might also threshold out a weak signal for an actual bar code bar. On the other hand, for a low contrast bar code, where the background paper is rough and off white or the ink is not dark enough, a low threshold might be employed because a weak bar code signal would be likely. In this instance, noise or bar code imperfections exceeding the low threshold may be detected as bar code intervals. A variety of other scanning circumstances effect the appropriate choice of threshold, but, in short, a single threshold is not well suited to addressing all of the typical real world variations.
Bar code scanners of the prior art typically include a single data recovery channel with a limited frequency range and a single threshold which may have both a DC and an AC component. Noise is typically limited by a single bandpass filter. This increases the likelihood that a label with severe imperfections will require repeated attempts to scan, or will fail to scan altogether, as the imperfect label is more likely to produce a response that falls outside the acceptable frequency range.
There exists, then, a need in the art for a bar code scanner having dual data recovery channels which satisfactorily limit or filter out noise but which provide a broader frequency range sufficient to decode labels having imperfections which produce frequencies at one end of the range or the other. Further, it is highly desirable to have a dynamic thresholding arrangement which is adjusted in real time to adapt to observed scanning conditions as described further below.
A video data recovery system according to the teachings of the present invention preferably includes first and second video signal generators for generating first and second video signals in response to light recovered from the reflection from a bar code. The data recovery system further includes first and second video signal receivers for receiving the first and second video signals, and first and second filters for limiting the frequency range of each of the first and second video signals, thereby producing first and second filtered video signals. The first and second filters limit the first and second video signals to differing but overlapping frequency ranges. The first and second video signals have differing thresholds applied. The first and second filtered and thresholded video signals are furnished to first and second analog-to-digital converters, respectively, to produce first and second digital data strings. If both of the first and second digital data strings contain useful data, the string containing higher-quality data is decoded to recover the bar code information. If only one of the first and second digital data strings contains useful data, that string is decoded. Further, the thresholds may advantageously set to aggressively threshold on one channel and to less aggressively threshold on the other channels with dynamic threshold adjusts being possible to adapt to the scan environment.